In the art of radiation sensing, it is often desirable to be able to determine the position of a beam of radiation or the position of a spot of light on a detector. For example, in my co-pending application Ser. No. 675,665, file Nov. 28, 1984, and assigned to the assignee of the present invention, I describe a system for determining the position of a surface for use in such applications as determining the placement of machine parts, materials, sheets, or objects without using tape measures or other normal contact devices. In this system a beam of light is transmitted to the surface whose position is to be sensed and the return beam of light falls on a detector as a spot of light. The position of this spot of light on the detector provides an indication of the position of the surface being sensed. Spot or beam position sensors used in the prior art and in my above referred to co-pending application comprise a position sensing photo-detector such as is manufactured and sold by United Detector Technology Company under the designation LSC-30D, for example. Such a detector consists of an elongated central photo-detector portion with two end portions to which electrical contact is made. When the spot of light is directly at the center of the detector, the current in the photo-detector divides evenly and the electron migration to the two ends is substantially equal, but when the spot is nearer to one end than the other, more current will flow to the closer end than to the further end and by analyzing the output from the ends of the detector an indication of the position of the spot may be obtained. These prior art detectors, sometimes referred to as "lateral effect photo-diodes" are quite temperature sensitive and their variance with temperature is rather unpredictable. Furthermore, the cost of lateral effect photo-diodes is also quite high and accordingly a need is created for a spot position sensor which is lower in cost, less sensitive to temperature and has a more predictable temperature variation from one sample to another.